Spinneret hole configuration for preventing bending of bicomponent extrudate

ABSTRACT

A spinneret ( 140 ) for extruding side-by-side bicomponent fibers includes a spinneret hole ( 148 ) having a cross-sectional shape transverse to the direction of polymer flow that is asymmetric with respect to the arrangement of the side-by-side streams of polymer components therein. The lower viscosity component flows through a portion of the spinneret hole having a higher perimeter-to-area cross-sectional shape than the portion of the spinneret hole through which the higher viscosity component through which the lower viscosity component flows. The increased surface area (i.e., cross-sectional perimeter) of the spinneret hole contacting the lower viscosity polymer flow compensates for the viscosity differential between the polymer components that would otherwise result in dogleg bending of the extrudate.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/134,263, entitled “Spinneret Hole Configurationfor Preventing Bending of Bi-Component Extrudate At Orifice,” filed May14, 1999. The disclosure of this provisional patent application isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for extrudingbicomponent, side-by-side polymer fibers whose components havesignificantly different viscosities and, more particularly, to spinnerethole configurations and arrangements of components therein that preventbending of side-by-side components of different viscosities uponextrusion.

2. Description of the Related Art

Woven and non-woven fabrics and yarns having desirable qualities can bemanufactured from crimped side-by-side, bicomponent synthetic polymerfibers. Such bicomponent fibers typically include two different polymersarranged as microfilaments or segments across the transverse crosssection of the fiber, which segments extend continuously along thelength of the fiber. A melt spinning process involving extrusion of themolten polymer from orifices of a spinneret can be used to form theseside-by-side bicomponent fibers. By causing one or both of theconstituent segments to crimp after extrusion, a fine denier fabric oryarn can be produced with improved characteristics, such as greaterbulkiness and softness, superior flexibility and drape, and betterbarrier and filtration properties for use in products such as disposableabsorbent articles, medical garments, filtration materials, apparel, andcarpet.

As is well known in the art, a side-by-side bicomponent or biconstituentfiber, in which the polymer components have significantly differentthermal shrinkage characteristics, will form helical crimps when thefiber is subjected to heat, as described in U.S. Pat. No. 5,093,061 toBromley et al., the disclosure of which is incorporated herein byreference in its entirety. A yarn made of side-by-side conjugatefilaments will also develop crimps if the yarn is stretched slightly andthen allowed to relax. A high degree of crimping of the bicomponentfibers is desirable, since a lofty or bulky non-woven fabric having goodsoftness, flexibility and drape characteristics and barrier propertiesresults.

Side-by-side bicomponent fibers can also be useful where one of thecomponents is used as an adhesive to bond the fibers into a web. In thiscase, one component typically has a significantly lower meltingtemperature than the other component and, upon heating to the softeningpoint, permits adjacent fibers to be bonded to each other withoutmelting the other component.

At present, common methods of producing side-by-side conjugate fibersemploy an arrangement that introduces two separated polymer streams, Aand B, into the spinneret orifice through narrow channels from oppositedirections above a spin hole. FIG. 1 illustrates a conventionalspinneret 10 having one channel 12 that directs a stream of polymer Adownstream and another channel 14 that directs a separate stream ofpolymer B downstream. Channels 12 and 14 respectively deliver polymers Aand B to the upstream end of a cylindrical counterbore 16 that tapers atits downstream end to a spinneret hole 18 forming an orifice 20 at thebottom face of spinneret 10. The spinneret hole 18 has a roundcross-section transverse to the flow direction, as shown in FIG. 2. Thepolymers form a side-by-side flow inside the spinneret hole 18 as wellas through the orifice 20, with each component occupying asubstantially. semi-circular transverse area within the spinneret hole.The fiber thus produced has a substantially round, side-by-sidetransverse cross section.

In prior art fiber melt spinning systems, the viscosities of the twoside-by-side polymer components, which are a function of temperature,must be matched. If the viscosities of two polymer components aredifferent, the higher viscosity polymer component flowing through thespinneret orifice loses more momentum than is lost by the low viscositycomponent This loss of momentum is due primarily to friction between thepolymer and the spinneret hole wall. Consequently, at the exit orifice,the low viscosity polymer component pushes the high viscosity componenttransversely and causes the combined polymer extrudate to bend ordeflect in the direction of the high viscosity polymer component. Thisbending phenomenon, shown in FIG. 3, is commonly referred to asextrudate dogleg. Extrudate with a high degree of dogleg can flow alongand contact the spinneret bottom surface, causing the combined polymercomponents to become, in effect, un-spinnable. Therefore, matching theviscosities of two polymers at the spin pack orifices has beenheretofore essential, limiting the permissible viscosity differencesand, thereby, the crimp that is obtained in the bicomponent fiber. Ithas been observed that drawn fibers formed from certain polymers withgreater viscosity differences exhibit a high degree of crimping. Thus,many desirable fibers formed of highly-crimpable polymer combinationsmay often be un-spinnable.

When the viscosities of the two polymers are equal at the spinningorifice, the polymer extrudate is straight and perpendicular to thedownstream spinneret surface; i.e., there is no bending or dogleg. Whenthe viscosities of the two polymers are different, the degree of bendingor dogleg is determined by: the viscosity difference; the spinneret holelength (or, the ratio of spinneret hole length to spinneret holediameter, L/D); the polymer flow rate through the orifice; and thevolume flow rate ratio between the two polymers. Bending of theextrudate increases with the increase of the viscosity difference, theorifice length and the polymer flow rate; bending can be increased ordecreased by varying the polymer flow rate ratio.

It is difficult to find pairs of polymers that yield the desired finalspun product and also have matched viscosities for a specified range ofspinning temperatures. For example, desirable polymers for formingside-by-side bicomponent fibers may include polyester (polyethyleneterepthalate or PET) and polybutylene terepthalate (or PBT). Due tolimited availability of commercial grades of these two polymers andother reasons (e.g., economical reasons), one must necessarily choose aPET and a PBT with slightly mismatched viscosities. Consequently, only alimited number of commercially available polymers have been usable toform crimpable side-by-side bicomponent fibers that yield fabrics andyams having the aforementioned highly desirable qualities. Accordingly,there remains a need for methods and apparatus capable of melt spinningside-by-side bicomponent fibers whose components have significantlydifferent viscosities.

SUMMARY OF THE INVENTION

Therefore, in light of the above, and for other reasons that becomeapparent when the invention is fully described, an object of the presentinvention is to provide processes and apparatus capable of compensatingfor viscosity differences between melt-spinnable polymers in order toprevent excessive bending of such polymers when extruded side-by-sidefrom orifices of a spinneret.

Another object of the present invention is to increase the number ofpolymer combinations available for forming side-by-side bicomponentfibers by expanding the acceptable range of polymer viscosity mismatchesthat will yield melt-spinnable fibers without excessive extrudatebending.

Yet another object of the present invention is to produce highlycrimpable side-by-side bicomponent fibers from pairs of polymers havingsubstantially mismatched viscosities.

Still another object of the present invention is to provide asymmetriccross-sectional geometries of side-by-side polymer streams within aspinneret hole that compensate for polymer viscosity differences,thereby preventing extrudate dogleg bending.

It is another object of the present invention to produce yarns, fabricsand textile products having improved characteristics from side-by-sidebicomponent fibers whose components have mismatched viscosities.

The aforesaid objects are achieved individually and in combination, andit is not intended that the present invention be construed as requiringtwo or more of the objects to be combined unless expressly required bythe claims attached hereto.

In accordance with the present invention, the aforementioneddifficulties associated with extruding side-by-side polymers to formbicomponent fibers are overcome by employing a spinneret holeconfiguration that is asymmetric with respect to the arrangement of thecomponent polymer streams flowing through and extruded from thespinneret hole. In particular, the components are arranged within thespinneret hole such that the ratio of the perimeter of the spinnerethole bounding the cross-sectional flow area of the lower viscositycomponent to the cross-sectional flow area of the lower viscositycomponent is greater than the corresponding ratio for the higherviscosity component. In other words, polymer viscosity differences thatnormally lead to dog-legging can be reduced or eliminated by arrangingthe polymer streams to increase the relative amount of surface area ofthe spinneret hole that contacts the lower viscosity component.

The transverse cross section shape of the spinneret hole and theresulting conjugate fiber may be trilobal, triangular, teardrop,bulb-and-stem or any other configuration that permits the lowerviscosity component to occupy a portion of the transverse cross-sectionhaving a greater perimeter-to-area ratio and hence permits the higherviscosity component to occupy a portion of the transverse cross-sectionhaving a lesser perimeter-to-area ratio. The spinneret hole geometriesand the associated asymmetric polymer component arrangements of thepresent invention advantageously allow heretofore un-spinnablecombinations of polymers with mismatched viscosities to be successfullymelt spun into crimpable side-by-side bicomponent fibers that can beformed into yarns and fabrics with superior characteristics, such asgreater bulkiness and softness, superior flexibility and drape, andbetter barrier and filtration properties for use in products such asdisposable absorbent articles, medical garments, filtration materials,apparel, and carpet.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing definitions, descriptions and descriptive figures of specificembodiments thereof wherein like reference numerals in the variousfigures are utilized to designate like components. While thesedescriptions go into specific details of the invention, it should beunderstood that variations may and do exist and would be apparent tothose skilled in the art based on the descriptions herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view in elevation of a spinneret holeof a conventional spinneret for producing a side-by-side bicomponentsynthetic polymer fiber.

FIG. 2 is a transverse cross-sectional view of the spinneret hole shownin FIG. 1.

FIG. 3 is a cross-sectional side view in elevation of a portion of aconventional spinneret illustrating the polymer bending or “dog-legging”phenomenon that occurs when the viscosities of the two polymercomponents differ significantly.

FIG. 4 is a diagrammatic side view in elevation of an assembly forextruding side-by-side bicomponent fibers in accordance with anexemplary embodiment of the present invention.

FIG. 5 is a cross-sectional side view in elevation of a spinneret holeof a spinneret for producing a side-by-side bicomponent syntheticpolymer fiber in accordance with an exemplary embodiment of the presentinvention.

FIG. 6 is a transverse cross-sectional view illustrating thedistribution of the higher and lower viscosity polymer componentsflowing through a spinneret hole having a substantially triangulartransverse cross-sectional shape in accordance with one embodiment ofthe present invention.

FIG. 7 is a transverse cross-sectional view illustrating thedistribution of the higher and lower viscosity polymer componentsflowing through a spinneret hole having a trilobal transversecross-sectional shape in accordance with another embodiment of thepresent invention.

FIG. 8 is a transverse cross-sectional view illustrating thedistribution of the higher and lower viscosity polymer componentsflowing through a spinneret hole having a teardrop transversecross-sectional shape in accordance with another embodiment of thepresent invention.

FIG. 9 is a transverse cross-sectional view illustrating thedistribution of the higher and lower viscosity polymer componentsflowing through a spinneret bole having a transverse cross-section inthe shape of a “U” with an undulating side wall corresponding to the topof the “U”, in accordance with another embodiment of the presentinvention.

FIG. 10 is a transverse cross-sectional view illustrating thedistribution of the higher and lower viscosity polymer componentsflowing through a spinneret hole having a keyhole-shaped transversecross-section in accordance with another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed explanations of FIGS. 4-10 and of the preferredembodiments reveal the methods and apparatus of the present invention.According to the present invention, two polymers with mismatchedviscosities, which could not be spun into side-by-side bicomponentfibers via a spinneret with conventional, round spinning orifices, canbe spun into side-by-side fibers with spinning orifices configured tohave a larger perimeter for the flow region of the lower viscositycomponent than for the flow region of the higher viscosity component.The present inventors have found that the flow speeds of the two polymercomponents can be substantially equalized (thereby avoiding extrudatebending) by increasing the proportion of the transverse cross-sectionperimeter of the spinneret hole that contacts the lower viscositycomponent and decreasing the proportion of the spinneret hole perimeterthat contacts the higher viscosity component. Accordingly, the presentinvention pertains to spinneret hole cross-sections that permit spinningof side-by-side fiber segments of different viscosities. As described ingreater detail hereinbelow, a variety of different spinneret holecross-sectional configurations and component distributions produceacceptable cross-sections for textile fibers from pairs of polymerhaving significantly different viscosities.

The present invention will generally be described in terms of methodsand apparatus that produce side-by-side bicomponent fibers having onlytwo segments or sub-fibers, one of each component. However, it should beunderstood that the scope of the present invention is meant to includeany fibers with two or more components or with two or more segments orsub-fibers where bending or dog-legging of the extrudate occurs withsubstantial polymer viscosity differences. For example, significantdog-legging would not normally be expected to occur with a bicomponentfiber having a transverse cross-section with four or more alternating“pie-shaped” wedges of the two components or with ribbon-shaped fiberhaving alternating higher and lower viscosity components. On the otherhand, a three component fiber having two lower viscosity components andone higher viscosity component could potentially experience unacceptableextrudate bending, depending on the cross-sectional arrangement of thecomponents; thus, the principles of the present invention can beextended to methods and apparatus for producing fibers having more thantwo segments and/or more than two components in appropriateconfigurations.

As used herein the term “extrudate” refers to the molten or semi-moltenpolymer streams that flow out of the orifices of a spinneret which, uponquenching and further processing (e.g., drawing) become fibers. The term“fiber” as used herein includes both fibers of finite length, such asconventional staple fiber, as well as substantially continuousstructures, such as filaments, unless otherwise indicated. The terms“segment” and “sub-fiber” refer to a portion of a fiber having acomposition that is distinct from the composition of another portion ofthe fiber, and the term “bicomponent” refers to a fiber having at leasttwo segments, wherein at least one of the segments comprises onematerial or component (e.g., a polymer), and the remaining segmentscomprise another, different material or component.

As used herein, the term “side-by-side” refers to an arrangement whereinthe transverse cross-section of the extrudate and subsequently formedfiber includes a first region formed of one component and a second,distinct region formed of another component, wherein the perimeter ofthe fiber cross-section is formed in part by an edge of the first regionand in part by an edge of the second region (e.g., side-by-side wouldnot include sheath-core arrangements). Typically, the fiber componentsare arranged so as to form distinct unocclusive cross-sectional segmentsor sub-fibers that retain their transverse cross-section shapesthroughout the length of the fiber.

Referring to FIG. 4, an assembly 100 for extruding side-by-sidebicomponent fibers in accordance with an exemplary embodiment of thepresent invention is shown. Apparatus 100 includes hoppers 112 and 114into which pellets of two different polymers, polymers A and B, arerespectively placed. Polymers A and B are respectively fed from hoppers112 and 114 to screw extruders 116 and 118 that melt the polymers. Themolten polymers respectively flow through heated pipes 120 and 122 tometering pumps 124 and 126, which in turn feed the two polymer streamsto a suitable spin pack 128 with internal parts for forming side-by-sidebicomponent fibers of a chosen cross-section.

Spin pack 128 includes a final polymer filtration system, distributionsystems and a spinneret 130 with an array of spinning orifices 132 whichshape the bicomponent fibers extruded therethrough. For example,orifices 132 may be arranged in a substantially horizontal, rectangulararray, typically from 1000 to 5000 per meter of length of the spinneret,with each orifice extruding an individual side-by-side bicomponentfiber. As used herein, the term “spinneret” refers to the lower mostportion of the spin pack that delivers the molten polymer to and throughorifices for extrusion into the environment. The spinneret can beimplemented with holes drilled or etched through a plate or any otherstructure capable of issuing the required fiber streams.

An array of side-by-side bicomponent fibers 134 exits the spinneret 130of spin pack 128, and the fibers are quenched as they enter theenvironment. A drawing force provided by an aspirator 136 (or othersuitable drawing mechanism, such as godets) is used to attenuate theextruded fibers. After drawing, the fibers may be processed in any of avariety of manners to form yarn or woven or non-woven fabric.

The polymer components are preferably incompatible or sufficientdissimilar to prevent substantial mixing of the components or chemicalreactions between the components. Specifically, when spun together toform a composite fiber, the polymer components preferably exhibit adistinct phase boundary between them so that substantially no blendpolymers are formed. The polymer components may comprise any one orcombination of melt spinnable resins, including, but not limited to,homopolymer, copolymers, terpolymers and blends thereof of: polyolefins,polyamides, polyesters, polyactic acid, nylon, poly(trimethyleneterephthalate), and elastomeric polymers such as thermoplastic gradepolyurethane. Suitable polyolefins include without limitation polymerssuch as polyethylene (e.g., polyethylene terephthalate, low densitypolyethylene, high density polyethylene, linear low densitypolyethylene), polypropylene (isotactic polypropylene, syndiotacticpolypropylene, and blends of isotactic polypropylene and atacticpolypropylene), poly-1-butene, poly-1-pentene, poly-1-hexene,poly-1-octene, polybutadiene, poly-1,7,-octadiene, andpoly-1,4,-hexadiene, and the like, as well as copolymers, terpolymersand mixtures of thereof. Further, the components may include additivessuch as dyes and/or pigments. For example, one component may include onepigment while the other component may include another pigment of adifferent color.

FIGS. 5 and 6 illustrate a spinneret hole of a spinneret 140 configuredto produce a side-by-side bicomponent fiber in accordance with anexemplary embodiment of the present invention. It will be understoodthat the spinneret includes an array of such spinneret holes tosimultaneously produce an array of side-by-side fibers. Spinneret 140includes channels 142 and 144 which respectively direct streams ofmolten polymers A and B to the upstream end of a counterbore 146 thattapers at its downstream end to a spinneret hole 148 forming an orifice150 at the bottom face of spinneret 140. The term “spinneret hole”describes the final capillary-like passage leading to the bottom face ofthe spinneret through which the side-by-side polymer components flowjust prior to being extruded into the environment. Polymers A and B flowside-by-side through counterbore 146, into the spinneret hole 148 andthrough orifice 150 into the environment.

As shown in FIG. 6, the spinneret hole 148 has a triangularcross-sectional shape transverse to the flow direction. Within thetriangular cross-section of the spinneret hole, the lower viscositypolymer component occupies a trapezoidal cross-sectional flow area atthe triangle base, bounded by two of the comers of the spinneret hole,while the higher viscosity component occupies a triangularcross-sectional flow area bounded by only one of the comers of thespinneret hole. In this exemplary configuration, the cross-sectionalarea (A) of the spinneret hole occupied by the higher viscositycomponent is the same as the cross-sectional area occupied by the lowerviscosity component; however, the portion of the perimeter of thespinneret hole bounding the trapezoidal-shaped lower viscosity componentis greater in length than the portion of the perimeter of the spinnerethole bounding the triangular-shaped higher viscosity component. By wayof non-limiting example, the area A occupied by each of the componentscan be 0.186 mm², with the perimeter of the lower viscosity componentbeing 2.33 mm and the perimeter of the higher viscosity component being1.97 mm. It has been discovered and experimentally verified by thepresent inventors that by increasing the perimeter of the lowerviscosity component, e.g., by employing an arrangement such as thatshown in FIG. 6, bending of the bicomponent extrudate can besubstantially reduced or eliminated.

The theory supporting the principle of the present invention can beunderstood from the following expression, the derivation of which ispresented hereinbelow:

P _(H) ² =P _(L) ²×(V _(L) /V _(H))×(F _(H) ² /F _(L) ²)  (1)

where:

P=flow area perimeter

V=polymer viscosity

F=polymer volumetric flow rate

_(L)=lower viscosity polymer component

_(H)=higher viscosity polymer component.

The expression in equation (1) defines the optimum parameterrelationship for minimizing or eliminating dog-legging in the extrudate.Variations from the optimum can be employed with small but acceptabledegrees of dog-legging. Thus, as used herein and in the claims inconnection with the relationship expressed in equation (1),“substantially equal” or “substantially maintaining the equality” meansthat any inequality in the relationship expressed in equation (1) issufficiently small to avoid the degree of extrudate bending ordog-legging that makes the fibers un-spinnable in practice (e.g.,contact of the extrudate with the spinneret face, clogging of thespinneret, clumping of the extrudate, contact between adjacent extrudatejust below the spinneret, etc.).

An explanation of this expression is as follows. In a round spin hole,the lower viscosity polymer component traverses the spinneret holefaster than the higher viscosity polymer component, since the pressuredrop through the spinneret hole is the same for both polymer components.Dog-legging or bending of the extrudate occurs at the spinneret holeexit because the faster, lower viscosity stream pushes toward theslower, higher viscosity stream causing a dog-leg bend or deflectiontoward the higher viscosity side. This phenomenon is analogous to abi-metal strip bending toward the metal component having the lowercoefficient of expansion upon heating.

The present invention comprises a spinneret hole cross-sectionalconfiguration that provides more wetted perimeter to the lower viscositypolymer flow region to create enough excess pressure drop to compensatefor the reduced viscosity. The greater the difference in the viscositiesof the two side-by-side polymer components, the greater the differencein wetted perimeter required. The expression of equation (1) allows fordifferences in flow rates of the two polymer components. The samepolymer combination may require different hole shapes for greatlydifferent flow rates.

The expression of equation (1) is derived as follows:

Variables:

S=speed of the polymer

P=flow area perimeter

V=polymer viscosity

F=polymer volumetric flow rate

L=spinneret hole length

A=cross section area of the flow path

H_(d)=hydraulic diameter of the flow path

_(L)=lower viscosity polymer component

_(H)=higher viscosity polymer component

The formula for the pressure drop through a spin hole is:

V×F×L/(A×H _(d) ²)  (2)

The pressure drop across the spinneret hole is the same for bothcomponents; therefore:

 V _(L) ×F _(L) ×L _(L)/(A _(L) ×H _(dL) ²)=V _(H) ×F _(H) ×L _(H)/(A_(H) ×H _(dH) ²);  (3)

L _(L) =L _(H); and  (4)

H _(d)=4×A/P.  (5)

Accordingly,

V _(L) ×F _(L)/(A _(L)×(16×A _(L) ² /P _(L) ²))×V _(H) ×F _(H)/(A_(H)×(16×A _(H) ² /P _(H) ²)),  (6)

which simplifies to:

V _(L) ×F _(L) ×P _(L) ² /A _(L) ³ =V _(H) ×F _(H) ×P _(H) ² /A _(H)³  (7)

Since the polymer velocities must be equal to avoid bending of theextrudate, and the velocity, S, is equal to F/A:

F_(L) /A _(L) =F _(H) /A _(H)  (8)

Equation (7) therefore simplifies to:

V _(L) ×P _(L) ² /A _(L) ² =V _(H) ×P _(H) ² /A _(H) ²  (9)

Arranging terms in equation (8) yields:

A _(L) =A _(H) ×F _(L) /F _(H),  (10)

and substituting A_(L) in equation (9) yields:

V _(L) ×P _(L) ²/(A _(H) ×F _(L) /F _(H))² =V _(H) ×P _(H) ² /A _(H)²  (11)

Canceling A_(H) ² from both sides of the equation yields:

V _(L) ×P _(L) ² ×F _(H) ² /F _(L) ² =V _(H) ×P _(H) ².  (12)

Solving for P_(H) ² yields the expression of equation (1), namely:

P _(H) ² =P _(L) ²×(V _(L) /V _(H))×(F _(H) ² /F _(L) ²)  (1)

Thus, to equalize the speeds of the higher and lower viscosity polymercomponents and thereby prevent bending of the bicomponent extrudate, theratio of the square of the perimeter for the flow area of the higherviscosity component to the square of the perimeter for the flow area ofthe lower viscosity component must be substantially equal to the ratioof the viscosity of the lower viscosity component to the viscosity ofthe higher viscosity component multiplied by the ratio of the square ofthe volumetric flow rate of the higher viscosity component to thevolumetric flow rate of the lower viscosity component. Stated insomewhat different terms, from equation (9) it can be seen that theratio of the perimeter to cross-sectional area of the low viscositycomponent must be greater than the ratio of the perimeter tocross-sectional area of the high viscosity component in order toeliminate bending of the extrudate.

As used herein, the term “spinneret hole perimeter” or simply“perimeter” refers to the closed curve bounding the transversecross-sectional area of the spinneret hole. The perimeters of the lowerviscosity component and higher viscosity component refer to portions ofthe spinneret hole perimeter contacting the respective components. Notethat the perimeters of the lower and higher viscosity components do notinclude the boundary between the components themselves, since it is theinteraction of the polymer flows with the spinneret hole side walls thataffects the flow velocity and extrudate bending.

Taking the special case where the cross-sectional flow path areas of thehigher and lower viscosity components are equal (making the volumetricflow rates the same when the component speeds are the same), it can beseen from equation (1) that the square of the perimeter for the flowarea of the higher viscosity component must be substantially equal tothe product of: the square of the perimeter for the flow area of thelower viscosity component; and the ratio of the viscosity of the lowerviscosity component to the viscosity of the higher viscosity component.In simplified terms, for a side-by-side streams of equal amounts oflower and higher viscosity components, bending of the extrudedcomponents can be prevented by making the perimeter of the lowerviscosity component greater than the perimeter of the higher viscositycomponent.

Conversely, to see that the lower viscosity polymer component will flowthrough the spinneret hole faster than the higher viscosity componentwhen the perimeter of the lower viscosity component is not increased,consider the case where the ratios of the perimeter to flow area of thetwo components are the same (P_(L)/A_(L)=P_(H)/A_(H)). Equation (7) thensimplifies to:

V _(L) ×F _(L) /A _(L) =V _(H) ×F _(H) /A _(H)  (13)

The polymer speed is equal to the volumetric flow rate divided by thecross-sectional flow area (S=F/A). Substituting this expression intoequation (13) yields

V _(L) ×S _(L) =V _(H) ×S _(H) or S _(L) /S _(H) =V _(H) /V _(L)  (14)

By definition, V_(L)<V_(H), so S_(H) (the speed of the higher viscositycomponent) must necessarily be less than SL (the speed of the lowerviscosity component) to maintain the equality in equation (14). Thus,when two polymers of different viscosities are extruded side-by-sidewith the equal cross-sectional areas and contacting equal amounts of thespin hole perimeter, the lower viscosity polymer will flow faster thanthe higher viscosity polymer, resulting in some degree of bending.

Referring again to FIG. 6, although the cross-sectional areas occupiedby the higher and lower viscosity components are the same within thespinneret hole, the triangular-trapezoidal arrangement of the twocomponents results in a greater perimeter for the lower viscositycomponent PL than for the higher viscosity component P_(H). When thepolymer component speeds are equal, so too are the flow rates (i.e., inthis case of equal areas); thus, it can be seen from equation (1) thatbending of the side-by-side extruded streams is prevented when therelationship of the perimeters PL and PH compensate for difference inthe polymer viscosities in accordance with the relationship:

P _(H) ² /P _(L) ² =V _(L) /V _(H)  (15)

In the example shown in FIG. 6, the flow cross-sectional areas A for thetwo polymer components are the same, while the perimeters P aredifferent. However, the areas need not be equal, depending on thepolymers and desired characteristics of the extruded fibers. In general,the ratio (and hence cross-sectional area occupied) of the twocomponents can vary. Preferably the weight ratio is in the range ofabout 10:90 to 90:10, more preferably from about 20:80 to about 80:20,and most preferably from about 35:65 to about 65:35. Likewise, theparticular perimeter values described depend on the desired product andpolymers employed.

Bending of the extruded side-by-side streams is eliminated when the twopolymer flow speeds are the same. Taking into consideration therelationship S=F/A (polymer flow speed=volumetric flowrate/cross-section flow area in the spinneret hole), under equal speedconditions, when the cross-section areas of the spinneret occupied bythe two polymer components differ, the flow rates of the componentsdiffer proportionally. It can be seen from equation (1) that differingflow rates of the two components impact the perimeter differentialrequired to compensate for a particular viscosity difference.

In cases where the selected perimeters of the polymer components do notentirely compensate for the viscosity difference between the polymercomponents (i.e., the relationship expressed in equation (1) issubstantially equal but not exactly equal), the residual deflection ordogleg bending of the extrudate can be compensated for or eliminated byorienting the flow of the quench air below the spinneret in thedirection opposite of the bending. That is, the quench air flows in thedirection from the high viscosity component toward the low viscositycomponent, substantially perpendicular to the boundary between the twocomponents. This air flow can counteract a limited degree of bending byurging the extrudate in the direction opposite the direction in whichthe extrudate tends to deflect as a result of the viscosity difference.

In general, the perimeter differential required to compensate forviscosity differences can be achieved by distributing the higherviscosity component to a portion of the spinneret hole cross-sectionalarea shaped to have high perimeter-to-enclosed-area ratio than thatoccupied by the lower viscosity component. As is well-known, a circleencloses a maximum area for a given perimeter, whereas an elongated,slot-shaped or jagged curve encloses a relatively small area for a givenperimeter. By locating the higher viscosity component in a more roundedor bulbous area of the spinneret hole and by locating the lowerviscosity component in a more elongated or jagged area of the spinnerethole, one can achieve a wide range of perimeter differentialscorresponding to a wide range of viscosity differences that preventextrudate dogleg. As can be seen in FIG. 6, the trapezoidalcross-sectional area occupied by the lower viscosity component issignificantly more elongated than the triangular cross-sectional areaoccupied by the higher viscosity component, providing the necessaryperimeter differential.

A number of other cross-sectional configurations provide significantperimeter differentials. Referring to FIG. 7, a trilobal spinneret holecross-sectional configuration in accordance with another embodiment ofthe present invention is shown. The higher viscosity component occupiesan entire lobe plus a small portion of each of the other two lobes, withthe lower viscosity component occupying the distal portions of the othertwo lobes. By way of non-limiting example, the cross-sectional areas ofthe higher and lower viscosity components can be the same (e.g., 0.16mm²), with the perimeters of the higher and lower viscosity componentsrespectively being 2.04 mm and 2.60 mm, respectively. Again, theparticular perimeter and area values provided are specific to aparticular embodiment and not intended to be limiting on the scope ofthe invention.

According to another embodiment of the present invention shown in FIG.8, the spinneret hole is teardrop-shaped in transverse cross-section,with the lower viscosity component occupying the narrower end andextending more than half way into the wider, more rounded end occupiedby the higher viscosity component. By way of non-limiting example, thecross-sectional areas of the higher and lower viscosity components canagain be the same (e.g., 0.22 mm²), with the perimeters of the higherand lower viscosity components respectively being 1.83 mm and 2.02 mm,respectively.

The configuration illustrated in FIG. 9, in accordance with anotherembodiment of the present invention, is generally U-shaped with thespinneret hole sidewall corresponding to the top of the “U” taking on ajagged shape to increase the perimeter in that region. The lowerviscosity component occupies the region bounded by the jagged wall,while the higher viscosity component occupies the area bounded by thesmooth, rounded wall. Upon extrusion, quenching and drawing, theconfiguration shown in this embodiment typically produces round fiber.

The spinneret hole of the embodiment illustrated in FIG. 10 has across-sectional shape resembling a mercury thermometer with one bulbousend and an opposite end in the form of an elongated stem. The stem endprovides a large perimeter and would typically contain the lowerviscosity component, with the higher viscosity component occupying thebulbous end.

This embodiment is useful for polymer components having vastly differentviscosities, thereby requiring significantly different perimeters forthe flow regions containing the respective polymer components.

From the foregoing examples, it can be seen that the present inventionencompasses spinneret hole configurations that are asymmetric withrespect to the boundary between the two polymer components. For example,the triangular-shaped spinneret hole cross-section shown in FIG. 6 wouldbe symmetric with respect to a polymer boundary line extending from onepoint of the triangle to the center of the opposite side; however, thetriangular shape is asymmetric with respect to the boundary line shownin FIG. 6 that is parallel to one of the sides of the triangle (i.e.,the boundary plane within the spinneret hole is parallel to one of thewalls of spinneret hole). Similarly the trilobal spinneret holecross-sectional shape is asymmetric with respect to the polymer boundarycurve shown in FIG. 7. Likewise, with each of the spinneret holetransverse cross-sectional shapes shown in FIGS. 8-10, the polymercomponents are arranged such that the spinneret hole shape is asymmetricwith respect to the orientation of the boundary between the twocomponents.

As those skilled in the art will appreciate, the relationship expressedin equation (1) is satisfied by a round spinneret hole extruding around, side-by-side bicomponent fiber in which the lower viscositycomponent is in the shape of a crescent. Such a fiber can be produced byappropriately controlling the flow of the two components into thespinneret hole or by employing a very long spinneret hole in which thetwo components will naturally take on this cross-sectional arrangement.However, such a fiber would have limited commercial value, since itwould exhibit poor crimpability relative to non-round side-by-sidefibers or conventional round fibers with symmetrically arrangedsegments. Moreover, while such a fiber could include a low melting pointcomponent useful as an adhesive for bonding fibers into a web, it wouldbe more economical to produce a sheath-core fiber for this purpose.

Depending on spinning, quenching and drawing conditions, theside-by-side fibers formed from the process and apparatus of the presentinvention will, to some degree, maintain the shape of the spinnerethole. For example, the triangular, trilobal, tear-shaped andstem-and-bulb spinneret holes will produce fibers with correspondingshapes. Thus, like the cross-sectional arrangement of the stream ofpolymer within the spinneret hole, the transverse cross-sectionaldistribution of the polymers in the resultant fibers will obey the basicrelationship that the ratio of the transverse cross-sectional perimeterto area of the component extruded with the lower viscosity (i.e., theviscosity within the spinneret hole at the spinning temperature) isgreater than the ratio of the transverse cross-sectional perimeter toarea of the component extruded with the higher viscosity. Furthermore,because the cross-sectional perimeter of the fiber is proportional tothe outer surface area of the fiber for a given length of fiber, itfollows that the ratio of the outer surface area to transversecross-sectional area of the component extruded with the lower viscosityis greater than the ratio of the outer surface area to transversecross-sectional area of the component extruded with the higherviscosity.

The specific embodiments illustrated and described herein are intendedto be exemplary and not limiting on the scope of the invention. Again,the present invention encompasses any spinneret hole cross-sectionalshape and arrangement of polymer components therein that yield aperimeter differential between the higher and lower viscosity componentssufficient to limit bending of the extrudate and permit successfulextrusion by adequately compensating for the viscosity differential ofthe components.

The prevent invention vastly expands the number of combinations ofcommercially available polymers that can be melt-spun into side-by-sidebicomponent fibers by allowing polymers having significantly differentviscosities, that were heretofore un-spinnable, to be co-extrudedwithout excessive dog-legging. In particular, combinations of polymersknown to yield highly crimped bicomponent fibers can now be produceddespite substantially viscosity differences between the components.These crimped fibers are useful in any product where properties such assoftness, strength, filtration or fluid barrier properties, and highcoverage at a low fabric weight are desirable or advantageous. Forexample, the fibers produced by the methods and apparatus of the presentinvention can be used in a variety of commercial products including, butnot limited to: softer diaper liners, sanitary napkins, disposable wipesor other disposable absorbent articles; medical fabrics having barrierproperties such as surgical gowns and drapes and sterilization wraps;filtration media and devices; and liners for articles of clothing (e.g.,a liner of a jacket).

The present invention is not limited to the particular apparatus andprocesses described above, and additional or modified processingtechniques are considered to be within the scope of the invention. Forexample, any number or combination of fiber processing techniques, yamforming techniques, and woven and non-woven fabric formation processescan be applied to the side-by-side fibers formed in accordance with thepresent invention.

Having described preferred embodiments of new and improved spinnerethole configuration for preventing bending of bicomponent extrudate, itis believed that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims. Although specificterms are employed herein, they are used in a generic and descriptivesense only and not for purposes of limitation.

What is claimed is:
 1. A melt spun side-by-side bicomponent fiber,comprising: a first component having a first viscosity at amelt-spinning temperature; and a second component arranged side-by-sidewith the first component and having a second viscosity higher than thefirst viscosity at the melt-spinning temperature; wherein a transversecross-section of the fiber includes an area of the first component andan area of the second component, a perimeter of the transversecross-section comprising a perimeter of the first component and aperimeter of the second component, and wherein the ratio of theperimeter of the first component to the area of the first component isgreater than the ratio of the perimeter of the second component to thearea of the second component.
 2. The fiber of claim 1, wherein at leastone of said first and second components is crimped.
 3. The fiber ofclaim 1, wherein said fiber has a non-round transverse cross-sectionalshape.
 4. The fiber of claim 3, wherein said fiber has a substantiallytriangular transverse cross-sectional shape.
 5. The fiber of claim 3,wherein said fiber has a substantially trilobal transversecross-sectional shape, including first, second and third lobes.
 6. Thefiber of claim 3, wherein said fiber has a substantially teardroptransverse cross-sectional shape having a narrow end and a wide end. 7.The fiber of claim 3, wherein the transverse cross-sectional shape ofsaid fiber includes a bulbous portion and an elongated stem portionextending from the bulbous portion.
 8. A yarn comprising a plurality ofthe fibers of claim
 1. 9. A fabric comprising a plurality of the fibersof claim 1.